1. Field of the Invention
The present invention relates to an optical head device and a near-field light emitting device, and more particularly to an optical head device which is suited to be used for a high-density optical memory (record/reading).
2. Description of Prior Art
In the art of optical memories which optically record and read information, in recent years, with speed-up of computer processing and development of multimedia, high-density devices which are capable of recording an extremely large volume of information are demanded, and in order to comply with the demand, a near-field optical recording technique is suggested. In a conventional optical memory using a laser beam, the recording density is limited depending on the diffraction limit of light, and such an optical memory can record and read marks of sizes of at least light wavelength (around 300 nm).
A recently proposed optical memory which uses near-field optics radiates light to a recording medium for recording/reading with the optical head and the recording medium arranged at an interval of only some dozen nanometers. At this time, the optical memory uses a fiber probe with a minuscule aperture smaller than light wavelength and/or an SIL (solid immersion lens) so that a light spot which is smaller than light wavelength can be formed on the recording medium. Thereby, in spite of the diffraction limit, it becomes possible to record and read minuscule marks less than 200 nm.
Incidentally, in the near-field optical technique, in order to obtain a minuscule light spot, it is preferred to converge light to the diffraction limit at a numerical aperture which is as high as possible. For this purpose, U.S. Pat. No. 5,125,750 suggested a method in which a convergent bundle of rays is directed to an SIL made of a material with a high refractive index by use of an objective lens.
In a conventional optical head device using an SIL, specifically, a laser beam is collimated into a parallel bundle of rays and is reflected by a mirror. Then, the laser beam is converged by an objective lens and is incident to the SIL. In this structure, it is necessary to use various optical elements such as a mirror, an objective lens, etc., which indispensably increases the size and weight of the optical head.
Lately, it is suggested to use an SIM (solid immersion mirror) instead of an SIL (see pages 59-62 of Reports of ODF'98, Tokyo, Jun. 16, 1998). The SIM converges light by performing refraction once and reflection twice in a material with a high refractive index, and this eliminates the necessity of using a convergent lens. However, a mirror to reflect a collimated laser beam is still necessary, and it is not perfect as a small and light optical head. Also, since the optical elements such as an SIM, a light source, a mirror, etc. must be assembled, troublesome works, namely optical axis adjustment and focusing must be carried out. Further, there are problems that chromatic aberration may occur because of refraction and that the focal length may shift with a change in temperature.